1. Field of the Invention
The present invention relates to an optical pickup apparatus for irradiating a light beam on an optical recording medium for recording and/or reproduction. More particularly, the present invention relates to an optical pickup apparatus for irradiating a light beam on an optical recording medium for recording and/or reproduction using a nonpolarization beam splitter.
2. Background of the Invention
Recently, with the amount of information increasing so much, there have been demands for even higher recording densities of storage in the optical discs used for packaging media for audio and visual information such as computer storage devices, compact discs, and video discs. To obtain such higher density optical discs, it is necessary to either shorten the wavelength .lambda. of the light beam emitted from the light source or else to increase the numerical aperture (NA) of the objective lens. This gives rises to several problems.
A conventional example will be first described with FIGS. 1 and 2.
FIG. 1 is a perspective view of a conventional optical pickup apparatus. FIG. 2 is a view of the optical path of the optical pickup apparatus. Reference numeral 1 denotes a laser beam generating device used as a light source. The laser beam generating device 1 has a semiconductor laser 1a, a lens 1b, a pair of reflectors 1c and 1d, a 1/4 wavelength plate 1e, a solid-state laser medium 1f, a non-linear optical crystal element 1g, a temperature control device 1h, and a cabinet 1i. The semiconductor laser 1a emits a pumping light beam for exciting the laser medium 1f. The lens 1b converges the pumping light beam radiated from the semiconductor laser 1a through the reflector 1c on the laser medium 1f. The pair of reflectors 1c and 1d constitute a resonator. The laser medium 1f and the nonlinear optical crystal element 1g are disposed in this resonator constituted by the pair of reflectors 1c and 1d. On one surface of the 1/4 wavelength plate 1e, that is, the surface facing the semiconductor laser 1a, is provided the reflector 1c. The reflector 1cexhibits wavelength selectivity such that it transmits through the pumping light beam having a wavelength, for example, 810 nm radiated from the semiconductor laser 1a while reflecting the light beam generated by the laser medium 1f having a wavelength, for example 1064 nm. The 1/4 wavelength plate 1e performs type II phase matching between the light beam emitted from the laser medium 1f i.e. the fundamental wavelength laser light beam and the second harmonic laser light beam generated by the non-linear optical crystal element 1g. For this type II phase matching, see U.S. Pat. No. 4,910,740. The laser medium 1f is a rod-shaped solid-state laser medium, such as Nd:YAG. The laser medium 1f is irradiated by the pumping light beam focused by the lens 1b through the reflector 1c. The laser medium generates the fundamental wavelength laser light beam. The non-linear optical crystal element 1g used is composed of KTP (KTiPO.sub.4). This non-linear optical crystal element 1g generates second harmonic laser light beam responsive to the fundamental wavelength laser light beam irradiated thereto. The reflector 1d is provided at one surface of the non-linear optical crystal element 1g. The reflector 1d exhibits wavelength selectivity such that it reflects the light beam of the fundamental wavelength, such as the light beam of a wavelength of 1064 nm, and transmits through the second harmonic laser light beam generated by the non-linear optical crystal element 1g, such as a light beam having a wavelength of 532 nm. The 1/4 wavelength plate 1e, the laser medium 1f, and the non-linear optical crystal element 1g are joined integrally with each other.
Reference numeral 2 denotes a mirror for deflecting the light beam to the upward direction. The mirror 2 deflects the second harmonic laser light beam outputted from the reflector 1d by 90.degree. and emits it toward the beam expander later explained.
Reference numeral 1h denotes a single temperature control device, for example, a thermo-electric (TE) cooler. The temperature control device 1h performs temperature control for the mounting plate where the semiconductor laser 1a and the lens 1b are mounted, the block to which the resonator is arranged, and the base member to which the mirror 2 is attached. Reference numeral 1i denotes a cabinet for accommodating the semiconductor laser 1a, the lens 1b, the resonator, the mirror 2, and the temperature control device 1h. At the top surface of the cabinet 1i is provided a window 1j for emission of the second harmonic laser light beam deflected 90.degree. by the mirror 2.
In the thus constituted laser beam generating device 1, the pumping light beam emitted from the semiconductor laser 1a is focused by the lens 1b and irradiated to the laser medium 1f through the reflector 1c and the 1/4 wavelength plate 1e. The fundamental wavelength laser light beam is generated by the laser medium 1f responsive to the irradiated pumping light beam. This fundamental wavelength laser light beam is irradiated on the nonlinear optical crystal element 1g. The non-linear optical crystal element 1g generates the second harmonic laser light beam based on the fundamental wavelength laser light beam irradiated on it. This second harmonic laser light beam is transmitted through the reflector 1d serving as the output surface and is deflected 90.degree. by the mirror 2. The second harmonic laser light beam deflected 90.degree. by the mirror 2 is outputted through the window 1j toward the beam expander described below.
Reference numeral 3 denotes a beam expander, which expands the diameter of the second harmonic laser light beam emitted from the laser beam generating device 1. This beam expander 3 is, for example, composed of a combination of two concave lenses.
Reference numeral 4 denotes a polarization beam splitter. The polarization beam splitter 4 separates the second harmonic laser light beam generated from the laser beam generating device 1 and the later mentioned reflected light beam reflected by the optical disc serving as the optical recording medium and deflects by 90.degree. the reflected light beam. Reference numeral 5 denotes a collimator lens which converts the second harmonic laser light beam expanded in diameter by the beam expander 3 into a parallel light beam. Reference numeral 6 is a 1/4 wavelength plate which converts the linear polarized light beam outputted from the laser beam generating device 1 into a circularly polarized light beam and converts the reflected light beam reflected by the optical disc into a linear polarized light beam once again from the circularly polarized light beam. The direction of polarization of the light beam emitted from the laser beam generating device 1 and the direction of polarization of the reflected light beam reflected by the optical disc differ, however.
Reference numeral 7 denotes an objective lens, which focuses the light beam passing through the 1/4 wavelength plate 6 on the recording surface of the optical disc. The objective lens 7 used is for example an aspherical single lens.
Reference numeral 8 denotes an optical disc used as the optical recording medium. The optical disc 2 includes a disc-shaped substrate of a thickness of 1.2 mm, as seen in a so-called compact disc, a recording layer provided on the surface of the disc-shaped substrate forming a recording surface 8a, and a protective layer provided on the recording layer. The recording layer is formed from a metallic material such as Al or Au in an exclusive read-only optical disc such as a compact disc and is formed from a phase changing type optical recording material or magneto-optical recording material in a recordable optical disc. The disc-shaped substrate, in the case of an exclusive read-only type optical disc, has the information signals recorded on it as indented pits on the recording surface and, in the case of a recordable optical disc, has the information signals recorded on it using changes in reflectivity or Kerr effect. The disc-shaped substrate is generally formed by a synthetic resin having optical transmittance, such as polyvinyl chloride (PVC), an acrylic resin such as polymethyl methacrylate (PMMA), and polycarbonate (PC). Among these, polycarbonate resin is superior as a material for the disc-shaped substrate in terms of its shock resistance, heat resistance, and dimensional stability at the time of injection molding and further is inexpensive in price.
Reference numeral 9 denotes a photodetector which receives the reflected light beam which has been polarized and separated by the polarization beam splitter 4.
As mentioned above, to increase the recording density of an optical disc, the technique has been used of increasing the numerical aperture (NA) of the objective lens 7 in addition to using a light source for emitting a light beam having a shorter wavelength.
If the numerical aperture (NA) of the objective lens 7 is made larger in order to obtain a high recording density optical disc, however, there is the problem that the birefringence of the disc-shaped substrate of the optical disc causes a degradation of the reproduced signal characteristics.
As mentioned above, in an optical disc with a disc-shaped substrate made of polycarbonate, the disc-shaped substrate generally has a biaxial birefringence (note that an explanation of the birefringence is given in Morikita Shuppan "Latest-Applied Physics Series 1: Crystal Optics", pp. 65 to 68 etc.)
In this case, in a polarization type optical system such as shown in FIGS. 1 and 2, while there was not that much of a problem with a system with a low numerical aperture (NA) of 0.45 or so (for example, a compact disc player system), if applied to a system with a high numerical aperture (NA) of about 0.6 (for example, a high recording density optical disc drive system), it was confirmed that the reproduced frequency characteristics fell significantly compared with the compact disc player system.
The birefringence in the thickness direction of the disc-shaped substrate of the polycarbonate forming the disc-shaped substrate of the optical disc is non-linear, so the state of polarization of the light beam of the portion corresponding to the outer circumference portion of the objective lens ends up changing. Therefore, it is not possible to detect all of the light beam reflecting from the optical disc by the polarization beam splitter. Namely, part of the reflected light beam is not emitted toward the photodetector, and part of the reflected light beam transmitted through the polarization beam splitter is returned to the light source. As a result, despite an objective lens with a high numerical aperture (NA) being used, the situation ends up equivalent to the reproduction of the optical disc using an objective lens with a small numerical aperture (NA). Due to this, the reproduced frequency characteristics deteriorate despite the use of the objective lens having the high numerical aperture (NA).
This will be explained in further detail using the results of measurements.
FIGS. 3A to 3D and FIGS. 4A to 4D show the results of measurement of the signal level with respect to the distribution of light intensity and spacial frequency (modulation transfer function: MTF) using optical discs having four types of birefringence.
As the birefringence appearing in the disc-shaped substrate of the optical disc, there are the in-plane birefringence defined as .DELTA.(Nx-Ny) and the perpendicular birefringence defined by .DELTA.(Nx-Nz) when the substrate surface is the XY-axes and the direction of thickness of the substrate is the Z-axis. An optical disc with a disc-shaped substrate made of glass as an ideal material with an in-plane birefringence and a perpendicular birefringence of both zero is shown in FIG. 3A and FIG. 4A, and an optical disc with a polycarbonate disc-shaped substrate with an in-plane birefringence .DELTA.(Nx-Ny) of a and a perpendicular birefringence .DELTA.(Nx-Nz) of b is shown in FIG. 3B and FIG. 4B. Further, an optical disc with a disc-shaped substrate made of polycarbonate with an in-plane birefringence .DELTA.(Nx-Ny) of -a and a perpendicular birefringence .DELTA.(Nx-Nz) of b is shown in FIG. 3C and FIG. 4C. FIG. 3D and FIG. 4D show an optical disc with a disc-shaped substrate made of polycarbonate with an in-plane birefringence .DELTA.(Nx-Ny) of zero and a perpendicular birefringence .DELTA.(Nx-Nz) of b.
Regarding the distribution of the light intensity, the light intensity is equally distributed in the case of an optical disc provided with a disc-shaped substrate made of glass with a birefringence of zero shown in FIG. 3A. As opposed to this, the total amount of light not only falls in the case of the optical discs shown in FIGS. 3B and 3C, but also the distribution of the light intensity in-plane becomes unbalanced and also the amount of light at the peripheral portions of the pupil partially falls, it will be understood. Further, in the case of the optical disc shown in FIG. 3D, since the in-plane birefringence is zero, not that great an imbalance in the distribution of the light intensity is observed in-plane, but the overall amount of light falls remarkably, it is understood.
Further, similar results are obtained for the reproduced frequency characteristics shown in FIGS. 4A to 4D. Compared with an optical disc with a birefringence of zero as shown in FIG. 4A, it is learned, the reproduced frequency characteristics, that is, the MTF, are degraded in each of the cases of FIGS. 4B to 4D. This is due to the fall in the amount of light at the peripheral areas of the pupil.
The distribution of the intensity of light caused by the biaxial birefringence of the disc-shaped substrate falls in this way because when the converged light passes through the optical disc, the phase delay (retardation) differs depending on the angle of incidence and the amount of the light of the periphery in the distribution of the light intensity after passing through the polarization beam splitter (after wave detection by the polarization beam splitter) falls. This phenomenon becomes more remarkable in effect the greater the numerical aperture (NA).
As mentioned earlier, when use is made of a laser beam generating device 1 as a light source for generating a second harmonic laser light beam, the second harmonic laser light beam transmitting through the polarization beam splitter 4 merely returns to the laser beam generating device 1, so there is no occurrence of mode hop noise due to the very slight amount of returning light beam returning to the light source as in the case of when a semiconductor laser is used as the light source, but the second harmonic laser light beam of the reflected light beam from the optical disc 8 and the second harmonic laser light beam emitted from the laser beam generating device 1 interfere. If the length of the optical path changes on the order of the wavelength, interference noise will be caused and the signal characteristics will degenerate. In this way, it was difficult in the conventional construction to deal with both birefringence and interference noise.
Therefore, if the conventional polarization optical system is used for a high recording density optical disc drive system, for example, a system using an objective lens with a high numerical aperture (NA=0.6), the biaxial birefringence of the disc-shaped substrate of the optical disc would have a detrimental effect on the reproduced signal characteristics and as a result the frequency characteristics would not be improved even if the numerical aperture (NA) were made larger, the crosstalk would increase, or the jitter would increase due to the intersymbol interference, resulting in the problem that the recording density would not in fact be improved.
Further, there was the problem that even if use was made of a nonpolarization optical system, there would be degradation of the signal characteristics due to so-called interference noise where the envelope of the reproduced signal would modulate in accordance with the flutter (vibration) of the optical disc.